The manufacture of cheese from milk concentrated by ultrafiltration (UF) is a radical innovation in cheesemaking which has been introduced to increase the yield of cheese. The increase in yield is largely due to incorporation into the cheese of soluble proteins, principally .alpha.-lactalbumin and .beta.-lactoglobulin. In conventional cheesemaking, these proteins are almost totally lost in the whey.
Historically, cheesemaking processes based on UF were first applied to the manufacture of soft, high-moisture cheeses. The well-known process by which soft cheeses are manufactured from retentate is based on the concentration of whole or skim milk by UF, with the addition of high-fat cream in the latter case, to yield a product ("pre-cheese") containing about 60% moisture and having a gross composition similar to that of the desired cheese. Coagulation and fermentation of the pre-cheese affords the finished cheese directly, with little or no whey drainage or loss of whey proteins. The rennet requirement is reduced by approximately 80%.
The manufacture of hard cheese from retentate presents difficulties much greater than those encountered in the manufacture of soft cheeses. These difficulties derive from the requirements of (i) removing more water from the retentate (typically 60% of moisture in 5:1 concentrated retentate) in hard cheese manufacture than is necessary in soft cheese manufacture in order to achieve the desired composition, while (ii) retaining the sensory attributes of the cheese variety and (iii) simultaneously achieving a sufficiently high yield increase to justify introducing the UF-based process.
Early attempts to develop manufacturing procedures for hard cheeses based on syneresis of rennet-coagulated retentate were characterized by substantial losses of fat and curd fines in the whey, excessive cheese moisture levels, and by poor cheese quality. More significantly, the cheese yield was increased little, if at all.
The difficulties with these early hard cheese manufacturing procedures were overcome and the basic requirements of a process for manufacturing hard cheese of acceptable quality and with satisfactory yield were established.
However, some of the steps and/or operations in this process had to be effected in a batch or semi-continuous fashion since no suitable apparatus was available for continuous processing, especially continuous coagulation.
One attempt at providing a simple apparatus for continuous production of coagulum from retentate is a single tube, continuously fed with a mixture of rennet and ripened retentate, and in which liquid movement takes place as plug flow. Devices based on these principles have been described (e.g. Grosclaude et al, U.S. Pat. Nos. 4,131,688 and 4,224,865). However, experience with a coagulation apparatus of this type showed it to be totally unsatisfactory in practice. Initially, the tube contained a zone of liquid ripened rennetted retentate, a zone in which coagulation was taking place, and a zone filled with coagulum. As time passed, however, coagulum formed and adhered onto the walls of the tube in the liquid-filled zone, and continuously built up and increased in thickness. Eventually, the tube became filled with a stationary annulus of coagulum surrounding a moving column of retentate which had insufficient residence time to coagulate. This experience showed that ripened rennetted retentate cannot be passed slowly through a tube, because of the build-up of coagulum on the walls, and suggested that coagulator designs based on a single tube could not be made to operate satisfactorily in a continuous mode.
It is therefore an object of the present invention to provide an apparatus which at least alleviates some of the problems associated with earlier apparatus for continuous production of hard cheese by providing an apparatus suitable for making hard cheese substantially continuously.
More specifically, the invention, in its preferred form, seeks to alleviate some of these problems by attempting to maximize the yield increase (and in particular to minimize fat and curd fines losses) and to ensure that the curd produced is suitable for further treatment in existing post-vat cheesemaking equipment and processes (e.g. Cheddaring, milling, salting, pressing in typical Cheddar cheese manufacture).
According to the present invention, there is provided an apparatus for the substantially continuous production of cheese from milk retentate, said apparatus comprising at least one coagulator tube having an inlet at one end for periodically receiving ripened rennetted retentate and an outlet at the other end for discharging coagulum formed in the tube said discharge being effected by pressure applied to, at or adjacent the inlet end, which is connected to a feed system comprising distribution means and circulation means, wherein said circulation means provides circulation of the retentate through the feed system under conditions whereby coagulation of the retentate is substantially prevented, and wherein the distribution means includes valve means arranged to periodically admit the retentate into the inlet of the tube from the distribution means. Typically there are two or more coagulator tubes; preferably 3, 4, 5, 6 or more similar such tubes.
Preferably, the feed system comprises a conduit in the form of a endless loop of closed piping or the like, which incorporates, as part of the loop, the distribution means and circulation means. One form of the distribution means is a manifold means which forms a part of the endless loop. However, the system may comprise, or include as part of the loop, one or more vessels, chambers or like reservoirs in fluid communication with the coagulator tube or tubes.
Preferably, the circulation means comprises a pump arranged for pumping the retentate at a sufficient velocity to prevent coagulum forming on the walls of the feed system or distribution means. Stirring or other agitation means may also be provided, if necessary or desired, particularly if any form of reservoir is included in the system.
Preferably, a separate pump, externally located to the feed system, provides within the distribution means the pressure required to effect injection of the retentate into the inlet(s) of the coagulator tube(s) and thereby to effect displacement of the coagulum that has formed in the tube from the previous injection of ripened rennetted retentate.
Preferably, there is a plurality of coagulation tubes, each of which are connected to the distribution means.
Preferably, the or all of the coagulation tube(s) are inclined at an angle to the horizontal. However, any orientation of the tubes relative to the remainder of the apparatus and/or to the floor of the factory in which it is located may be used.
Preferably, there is a cutting assembly located at or adjacent the outlet or outlets for cutting the coagulum into appropriately sized pieces, usually cubes or cuboids of about 1cm.sup.3, as or after it emerges from the coagulation tube(s). Most conveniently this can be done in two stages by providing slicing means located at or near the outlet(s) of the coagulator tube(s), to slice the coagulum longitudinally into substantially square section strands, and cutting means located adjacent to the slicing means to cut the coagulum strands substantially transversely to form cubes.
The present invention thus provides an apparatus for the substantially continuous production of cheese curd suitable for subsequent manufacture into hard cheese (such as Cheddar or related types) with minimal losses of milk solids in any form. More particularly, the apparatus of the present invention is suited to the coagulation and cutting of ripened retentate as required by the process described in Australian Patent Application No. PF 6207/82.
The design of the apparatus of the present invention generally includes consideration of the following steps and operations during the process of making hard cheese from retentate.
(1) Mixing of ripened retentate with rennet in a suitable amount at a temperature approximating the desired coagulation temperature, with sufficient agitation to ensure thorough mixing. PA1 (2) Keeping the thus ripened rennetted retentate vigorously agitated in the distribution means, preferably the endless closed loop, with sufficient agitation so the closed loop is substantially free of coagulum. The mean residence time of the ripened rennetted retentate in the distribution means being sufficiently short that it has no adverse affect on losses of fat and curd fines in subsequent cheesemaking. The function of the distribution means is to hold the ripened rennetted retentate preparatory to its distribution to the coagulator tube(s) in a defined sequence. PA1 (a) Segment sealing--excessively short segments (low L/D) may tend to distort and allow the incoming liquid retentate to leak past the partially coagulated segment which is being displaced. PA1 (b) Loss of plug flow--at very low L/D the partially coagulated segment may become quite unstable and may tilt or rotate within the tube. Also bulging of the segment centre can occur due to a combination of the pressure applied via the incoming retentate (or other means) and the frictional drag between the tube walls and the moving segment of coagulum. This may result in the extrusion of insufficiently coagulated retentate. PA1 (c) Back pressure--with high L/D values excessive pressures may need to be applied via the incoming rennetted retentate to displace the coagulating segment. PA1 (d) Frictional drag--with high L/D the consequent surface area/volume ratio may result in excessive frictional drag between the tube walls and the moving segment of partially coagulated retentate. This drag could result in disturbance of the coagulum, especially in the outer regions of the segment cross-section. PA1 (e) Cutting rate--high L/D values may result in high linear velocity of coagulum issuing from the coagulation tube. In turn, this requires a high speed cutting operation, leading to the application of excessive forces to the coagulum. PA1 (a) Quantities of 10 mm cubes of coagulum were agitated in drums of different diameters after cutting and then made into cheese. Drum diameters of 0.75 m and 1.2 m respectively were used and the levels of fat observed in the respective whey samples were 2.3 and 4.8% respectively. PA1 (b) Passage of coagulum around a sharp 90.degree. bend caused increased losses of fat in whey relative to a control (7.0% as compared to 4.6%).
In an alternative procedure, steps (1) and (2) may be combined ie. mixing of rennet, fermented retentate (as defined in Australian Patent Application PF 6207/82) and fresh retentate may take place in the vigorously agitated chamber.
In the preferred embodiment of the apparatus, the ripened rennetted retentate is transferred from the distribution means to a plurality of coagulator tubes. Transfer takes place to one or some of the total number of tubes at a time, and the tube(s) may be fractionally (1/2, 1/3, 1/4, 1/5, etc.) or completely filled in a given filling operation. The portion of tube filled in such a filling operation is termed a "segment" and the time taken is termed the "segment filling time".
The tubes should preferably be maintained at or near the desired coagulation temperature, such as for example up to .+-.10.degree. C. of the coagulation temperature of the coagulum. The coagulation tubes may be divided into two or more zones, each zone at the same or at different temperatures. There may be a temperature difference, such as a gradual temperature gradient, between the inlet and outlet end of the tubes. One way of maintaining the tubes at the desired temperature is to have the tubes surrounded by water jackets for receiving continuously circulated water at a selected and/or predetermined temperature.
The tubes are filled in a defined sequence. Transfer of the liquid feedstock occurs under pressure via piping and inlet valves, forming part of the feed system, located at the base of each tube. The piping, and as far as possible the valves, are arranged so that the ripened rennetted retentate they contain can be kept vigorously agitated, as required in (2) above.